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charset="utf-8" Adding documentation on landing pad aka indirect branch tracking on riscv and kernel interfaces exposed so that user tasks can enable it. Signed-off-by: Deepak Gupta --- Documentation/arch/riscv/zicfilp.rst | 104 +++++++++++++++++++++++++++ 1 file changed, 104 insertions(+) create mode 100644 Documentation/arch/riscv/zicfilp.rst diff --git a/Documentation/arch/riscv/zicfilp.rst b/Documentation/arch/risc= v/zicfilp.rst new file mode 100644 index 000000000000..23013ee711ac --- /dev/null +++ b/Documentation/arch/riscv/zicfilp.rst @@ -0,0 +1,104 @@ +.. SPDX-License-Identifier: GPL-2.0 + +:Author: Deepak Gupta +:Date: 12 January 2024 + +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D +Tracking indirect control transfers on RISC-V Linux +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D + +This document briefly describes the interface provided to userspace by Lin= ux +to enable indirect branch tracking for user mode applications on RISV-V + +1. Feature Overview +-------------------- + +Memory corruption issues usually result in to crashes, however when in han= ds of +an adversary and if used creatively can result into variety security issue= s. + +One of those security issues can be code re-use attacks on program where a= dversary +can use corrupt function pointers and chain them together to perform jump = oriented +programming (JOP) or call oriented programming (COP) and thus compromising= control +flow integrity (CFI) of the program. + +Function pointers live in read-write memory and thus are susceptible to co= rruption +and allows an adversary to reach any program counter (PC) in address space= . On +RISC-V zicfilp extension enforces a restriction on such indirect control t= ransfers + + - indirect control transfers must land on a landing pad instruction `lpad= `. + There are two exception to this rule + - rs1 =3D x1 or rs1 =3D x5, i.e. a return from a function and returns are + protected using shadow stack (see zicfiss.rst) + + - rs1 =3D x7. On RISC-V compiler usually does below to reach function + which is beyond the offset possible J-type instruction. + + "auipc x7, " + "jalr (x7)" + + Such form of indirect control transfer are still immutable and don't r= ely + on memory and thus rs1=3Dx7 is exempted from tracking and considered s= oftware + guarded jumps. + +`lpad` instruction is pseudo of `auipc rd, ` with `rd=3Dx0`` an= d is a HINT +nop. `lpad` instruction must be aligned on 4 byte boundary and compares 20= bit +immediate withx7. If `imm_20bit` =3D=3D 0, CPU don't perform any comparisi= on with x7. If +`imm_20bit` !=3D 0, then `imm_20bit` must match x7 else CPU will raise +`software check exception` (cause=3D18)with `*tval =3D 2`. + +Compiler can generate a hash over function signatures and setup them (trun= cated +to 20bit) in x7 at callsites and function prologues can have `lpad` with s= ame +function hash. This further reduces number of program counters a call site= can +reach. + +2. ELF and psABI +----------------- + +Toolchain sets up `GNU_PROPERTY_RISCV_FEATURE_1_FCFI` for property +`GNU_PROPERTY_RISCV_FEATURE_1_AND` in notes section of the object file. + +3. Linux enabling +------------------ + +User space programs can have multiple shared objects loaded in its address= space +and it's a difficult task to make sure all the dependencies have been comp= iled +with support of indirect branch. Thus it's left to dynamic loader to enable +indirect branch tracking for the program. + +4. prctl() enabling +-------------------- + +`PR_SET_INDIR_BR_LP_STATUS` / `PR_GET_INDIR_BR_LP_STATUS` / +`PR_LOCK_INDIR_BR_LP_STATUS` are three prctls added to manage indirect bra= nch +tracking. prctls are arch agnostic and returns -EINVAL on other arches. + +`PR_SET_INDIR_BR_LP_STATUS`: If arg1 `PR_INDIR_BR_LP_ENABLE` and if CPU su= pports +`zicfilp` then kernel will enabled indirect branch tracking for the task. +Dynamic loader can issue this `prctl` once it has determined that all the = objects +loaded in address space support indirect branch tracking. Additionally if = there is +a `dlopen` to an object which wasn't compiled with `zicfilp`, dynamic load= er can +issue this prctl with arg1 set to 0 (i.e. `PR_INDIR_BR_LP_ENABLE` being cl= ear) + +`PR_GET_INDIR_BR_LP_STATUS`: Returns current status of indirect branch tra= cking. +If enabled it'll return `PR_INDIR_BR_LP_ENABLE` + +`PR_LOCK_INDIR_BR_LP_STATUS`: Locks current status of indirect branch trac= king on +the task. User space may want to run with strict security posture and woul= dn't want +loading of objects without `zicfilp` support in it and thus would want to = disallow +disabling of indirect branch tracking. In that case user space can use thi= s prctl +to lock current settings. + +5. violations related to indirect branch tracking +-------------------------------------------------- + +Pertaining to indirect branch tracking, CPU raises software check exceptio= n in +following conditions + - missing `lpad` after indirect call / jmp + - `lpad` not on 4 byte boundary + - `imm_20bit` embedded in `lpad` instruction doesn't match with `x7` + +In all 3 cases, `*tval =3D 2` is captured and software check exception is = raised +(cause=3D18) + +Linux kernel will treat this as `SIGSEV`` with code =3D `SEGV_CPERR` and f= ollow +normal course of signal delivery. --=20 2.45.0